There are many applications where it is necessary to remove a liquid from a mixture of solids and liquids. The solids are typically suspended in the liquid or, in low liquid concentrations, the liquid may be bound to the solids, e.g. absorbed by the solids.
A common example of these applications is in the recycling of liquid and solids from industrial by-products or waste. Here, the liquid may be used as a transport medium for the solids or used in the processing of a product, e.g. as a coolant/lubricant for machine apparatus. Alternatively, the liquid may be present as part of the product or waste, e.g. water in organic waste.
This waste typically forms as a ‘slurry’ or ‘sludge’. A ‘sludge’, as referred to herein, has a higher concentration of solids in suspension than a corresponding ‘slurry’.
One method of solid-liquid separation is to deposit the mixture in settling ponds where the suspended solids settle into defined layers over time depending on their density relative to the liquid. Sewerage treatment facilities and an ash slurry from a coal-fired boiler installation use such a system.
Prior to processing, the solids from these mixtures, the water content must be significantly reduced, e.g. from around eighty percent by weight to less than twenty percent. The slurry is thus transformed into a lower volume, damp, semi-solid sludge that is easier to handle and lighter to transport, or which can be more easily processed.
There are many existing techniques for removing liquid from a solid-liquid mixture and the following examples are typical of the prior art:                Evaporation or heat extraction—In this process the mixture is heated to evaporate the liquid from the solids. The evaporated liquid is then condensed and recycled.        Centrifugal—The mixture can also be placed in a centrifuge chamber with a liquid-permeable filtrate about a periphery thereof. On activation of the centrifuge, the liquid thus passes through the filtrate and the solids can then be removed from the chamber.        Compression—The mixture may be compacted by being passed between two conveyor belts with a constricting space therebetween, the liquid being forced from the solid.        Filtration—The mixture may be passed over a filtrate medium or grating that is liquid-permeable so as to allow the liquid to drain from the mixture.        
It will be clear that each of the aforementioned methods have attendant advantages and disadvantages in processing different mixture-types, e.g. compression techniques may not be suitable for mixtures with a low solids concentration and a centrifugal method may not be suitable where the solids are sharp and abrasive.
The following description will be focused on filtration methods which prove most useful in a variety of mixture-types.
There are a number of different filtration techniques that can be used, though they generally comprise passing the mixture over one side of a liquid-permeable filtrate membrane and then applying a force to press the mixture against the filtrate to force the liquid therethrough. The force may be applied passively e.g. gravity, and/or actively, e.g. by a vacuum on the opposing side of the filtrate or a compression chamber on the mixture side. The filtrate may also be agitated to encourage liquid separation.
Known types of such vacuum filtrate systems are described in U.S. Pat. Nos. 4,137,169 and 4,880,538 by El Hindi, U.S. Pat. No. 4,154,686 by Ootani et al., U.S. Pat. No. 7,334,688 by Pahl et al. and U.S. Pat. No. 6,622,870 by Prinssen. Similar systems are also described in Canadian Patent No. 991094 by van Oosten and PCT publication No. WO2001/097948 by Marchal. French patent No. 2,787,035 by Benacchio describes another vacuum filtrate system.
The Thissen, Prinssen and Marchal devices are all similar systems that generally comprise a conveyor system onto which the mixture to be filtered is deposited. The conveyor system has two rollers about which an endless supporting belt is located to provide a movable supporting area between the rollers. The belt supports a filter medium which also passes about the rollers. The belt has a number of apertures allowing passage of liquid through the filter medium and below the belt. The belt is also formed with a series of transverse troughs which help guide the mixture toward the apertures situated in the troughs. A vacuum chamber (or “suction box”) is placed under the belt between the rollers and forms a pressure gradient through the cloth and belt to suck the liquid from the mixture via the cloth and apertures. The vacuum chamber of Prinssen, Thissen and Marchal systems is configured to reciprocate between the rollers to move with the belt in order to maximise the time of suction in each cycle.
The Thissen and Prinssen devices also use a compression chamber on the opposing side of the cloth to the vacuum in order to provide a greater pressure gradient, or in the Prinssen system, pressurise the entire apparatus to prevent gasification of mixture components.
While such prior art systems may be effective at achieving a high reduction in water content in most applications, there are a number of inherent disadvantages in using such complex systems, as described below.
Belt-based systems such as the Thissen, Prinssen and Marchal systems can prove expensive as the supporting rubber belts used often need replacing as they become stretched or damaged under the large stresses caused by the vacuum and weight of the mixture. Replacing belts can prove costly in terms of materials and operational delay.
Moreover, such systems typically only apply the vacuum via the apertures in the belts, which can be small or narrow, thus potentially not evacuating liquid evenly through the filter medium.
In contrast, the Pahl et al. and El-Hindi systems do not use rubber belts and instead use a chain mesh belt or screen with the filter medium only passing over the upper surface of the screen. Thus, the problem of confined suction and replacing belts is somewhat mitigated.
However, the vacuum chamber is inherently more difficult to seal against the mesh screen of the El-Hindi systems and thus requires a complicated sealing mechanism in order to function properly. Such sealing mechanisms are described in U.S. Pat. No. 4,880,538 by El-Hindi, U.S. Pat. No. 4,147,635 by Crowe and U.S. Pat. No. 7,334,688 by Pahl et al.
It is therefore an object of the present invention to provide a liquid removal apparatus which will at least go some way towards overcoming the above-mentioned problems, or at least provide the public with a useful choice.
It should be appreciated that the discussion of the references herein states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of any cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein; this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.